Hyperbranched polymers are classified as dendritic polymers together with dendrimers. While related-art polymers generally have a string form, these dendritic polymers have these own specific structures in a respect of introducing a branched structure positively. Accordingly, these dendritic polymers have various characteristics such as having a nanometer size, having surfaces capable of retaining many functional groups, being capable of having a lower viscosity than linear polymers, exhibiting a behavior like fine particles with little entanglement between molecules, and being amorphous to be able to control their solubility in a solvent, so that expectations lie in applications utilizing these characteristics.
Particularly, the most remarkable characteristic of dendritic polymers is a large number of terminal groups. The more the molecular weight is, the more the number of branched chains increases, therefore, the absolute number of terminal groups becomes larger as the molecular weight of dendritic polymers increases. In such a dendritic polymer having a large number of terminal groups, intermolecular interactions depend largely on the types of the terminal groups, resulting in large variations in the glass transition temperature, the solubility, the thin film forming property, or the like. Accordingly, such a dendritic polymer has characteristics that no general linear polymer has. Further, when reactive functional groups are added to such a dendritic polymer as terminal groups, the dendritic polymer have the functional groups with an extremely high density, therefore its applications as, for example, a high sensitive scavenger for functional substances, a high sensitive multifunctional crosslinking agent, a dispersant for metals or metal oxides, or a coating agent, are expected.
An advantage of the hyperbranched polymer over the dendrimer is in its simplicity for synthesis, which is advantageous particularly in an industrial production. Generally, while the dendrimer is synthesized by repeating protection and deprotection, the hyperbranched polymer is synthesized by a one-step polymerization of a so-called ABx type monomer having in one molecule, a total of three or more substituents of two types.
As a synthesis method of a hyperbranched polymer, a method for synthesizing the hyperbranched polymer by a living radical polymerization of a compound having a vinyl group while having a photo polymerization initiating ability, is known. For example, a synthesis method of a hyperbranched polymer by a photo polymerization of a styrene compound having a dithiocarbamate group (see Non-Patent Documents 1, 2 and 3), and a synthesis method of a hyperbranched polymer having a dithiocarbamate group by a photo polymerization of an acrylic compound having a dithiocarbamate group (see Non-Patent Documents 4, 5 and 6), are known. However, when these hyperbranched polymers are applied in optical fields, since a technology for precisely controlling the refractive index of the hyperbranched polymer becomes necessary, a hyperbranched polymer in which the refractive index is precisely controlled while retaining its hyperbranched structure, has been desired. In addition, since these hyperbranched polymers have in the molecule thereof, a dithiocarbamate group having a photo polymerization initiating ability, they remain in a living state relative to light and do not have high thermal stability. Thus, an optically and thermally stable hyperbranched polymer having no dithiocarbamate group has been desired.    [Non-Patent Document 1]    Koji Ishizu, Akihide Mori, Macromol. Rapid Commun. 21, 665-668 (2000)    [Non-Patent Document 2]    Koji Ishizu, Akihide Mori, Polymer International 50, 906-910 (2001)    [Non-Patent Document 3]    Koji Ishizu, Yoshihiro Ohta, Susumu Kawauchi, Macromolecules Vol. 35, No. 9, 3781-3784 (2002)    [Non-Patent Document 4]    Koji Ishizu, Takeshi Shibuya, Akihide Mori, Polymer International 51, 424-428 (2002)    [Non-Patent Document 5]    Koji Ishizu, Takeshi Shibuya, Susumu Kawauchi, Macromolecules Vol. 36, No. 10, 3505-3510 (2002)    [Non-Patent Document 6]    Koji Ishizu, Takeshi Shibuya, Jaebum Park, Satoshi Uchida, Polymer International 53, 259-265 (2004)